1. Field of the invention
The invention relates to electrical power supplies used for powering X-ray tubes, and more particularly, in such power supplies, it relates to means for supporting and cooling the various electric circuit components.
2. Description of the prior Art
An X-ray tube comprises a filament type cathode which emits a beam of electrons towards an anode or anticathode. The anode is made of a substance such as tungsten or molybdenum which emits X-rays when bombarded by the electron beam from the cathode. In order to obtain a beam of high-energy electrons, the electrons are accelerated by an intense electric field set up between the cathode and the anode. To this end, the anode is raised to a positive potential of several tens of kilovolts relative to the cathode, and this potential may exceed 100 kilovolts and may even reach 140 kilovolts.
Such voltages are provided by high-voltage power supplies which, as shown in FIG. 1, comprise a transformer 10 which is connected to rectifier and voltage-doubler circuits More precisely, the transformer 10 has a single primary winding 12 to which an alternating voltage is applied, and a secondary circuit 13 which is connected to the rectifier and voltage- doubler circuits In conventional manner, each rectifier and voltage-doubler circuit 11 consists in a single secondary winding 14, two diodes D1 and D2, and two capacitors C1 and C2 which are interconnected as shown in FIG. 1. Each rectifier and voltage-doubler circuit is connected to the following such circuit so that their output voltages are added together, thereby obtaining a very high voltage on the last voltage-doubler circuit of the assembly.
More precisely, the transformer comprises a primary winding 12 and twelve secondary windings S1 to S12, with secondary windings S1, S5, S6, and S12 being shown in the figure. Similarly, it includes twenty-four identical rectifier diodes D1 to D24, with components D1, D2, D3, . . . , D12, D13, D14, . . . , D22, D23, and D24 being shown in the figure.
The circuit also includes twenty-four filter capacitors C1 to C24 with capacitors C1, C2, C3, . . . , C12, C13, C14, . . . , C23, and C24 being shown in the figure.
Each of the secondary windings S1 to S12 has two output terminals. Taken together, the output terminals have references B1 to B24, but only the following terminals are shown in the figure: B1, B2, B3, . . . , B5, B6, B7, B8, . . . , B23, and B24.
In FIG. 1 the common point between capacitor C1 and diode D1 constitutes the high-voltage output terminal HT via a resistor R, and the common point between capacitor C24 and diode D24 constitutes the ground output terminal which is associated with a discharge gap 9.
In order to measure the amplitude of the high-voltage the output terminal HT is connected to a measurement circuit (not shown) connected to a point M via a resistor R' and a variable capacitor C'. The point M is connected to ground by a discharge gap 9'.
In a typical embodiment, each rectifier and voltage-doubler circuit has an output voltage of 6 kilovolts such that the voltage at the output from the twelfth rectifier and voltage-doubler circuit is seventy-two kilovolts.
It may be observed that in order to obtain a potential difference of about 140 kilovolts between the cathode and the anode of an X-ray tube, the cathode is connected to a negative potential of 70 kilovolts relative to ground while the anode is connected to a positive potential of 70 kilovolts relative to ground. To this end, two power supply circuits are used, both being identical to that shown in FIG. 1.
It will be understood that making a high-tension power supply as shown in FIG. I gives rise to insulation problems which are often solved by keeping conductors that are at very different potentials well apart from one another and by interposing an insulating medium between them such as oil, which medium a)so serves as a cooling liquid. The resulting apparatuses are large and bulky.
Further, X-ray tubes are being used more and more frequently under pulse conditions at ever increasing repetition rates. In the circuit of FIG. 1, this means that the primary winding is fed with an alternating voltage at a high frequency, of the order of several tens of kilohertz. Under such new operating conditions, the performance of the circuit shown in FIG. 1 is limited by the stray inductance and capacitance of the conductors and of the transformer windings whose values are difficult to establish and to compensate.
In French patent application number 89 01357, filed Feb. 2, 1989 and entitled "A high-voltage power supply for an X-ray tube", the Applicant describes a power supply in which the relative positions of the various components tend to minimize stray inductance and capacitance, and contribute to reducing the overall bulk of the assembly while being very easy to assemble.
Further, by making the secondary circuit in the form of concentric windings, it is only the stray capacitance between the first secondary winding and ground which has any effect while the other stray capacitances between the various secondary windings have no effect since they are at an A.C. voltage.
In order to restrict the lengths of the connection conductors interconnecting the output terminals B1 to B24 of the secondary windings S1 to S12 firstly to the diodes D1 to D24 and secondly to the capacitors C1 to C24, the invention of the above-mentioned patent application provides firstly for making the secondary windings so that their similar odd-number terminals B1, B3, . . . , B23, are disposed on a firs& side of the windings while their even-numbered output terminals B2, B4, . . . , B24 are disposed on the other or second side of the secondary windings.
It is then provided that the diodes D1 to D24 are installed on a common support which is disposed on the same side as output terminals B1, B3, . . . , B23 of the secondary winding. The capacitors C1 to C24 are provided on the outer periphery of the secondary windings and their connections are made firstly to the diodes D1 to D24 on the first side of the secondary windings, and secondly to the output terminals B2, B4, . . . , B24 on the second side of the secondary windings. This particular disposition of the various components will be better understood with reference to the description of FIGS. 2 and 3 in which items that are identical to those shown in FIG. 1 have the same references.
The power supply comprises two half-shells 20 and 21 in which housings are provided for receiving the primary winding 12' the secondary windings S1 to S12, the capacitors C1 to C24, and the diodes D1 to D24. To this end, each of the half-shells 20 (or 21) has three annular compartments 22, 23, and 24 (or 26, 27, and 28) disposed around a cylindrical central portion 25 (or 29).
The first annular compartment 22 (or 26) is at the periphery of the central portion 25 (or 29), whereas the second annular compartment 23 (or 27) is at the outer periphery of the first compartment 22 (or 26). The third compartment 24 (or 29) is disposed to one side of the first two compartments 22 and 23 (or 26 and 27) and is separated therefrom by respective partitions 30 and 31 (or 32 and 33) which are pierced by orifices.
The central portions 25 and 29 are designed to receive, in particular, the primary winding 12 and one of the branches 34 of the magnetic circuit 35 of the transformer 10. The first annular compartments 22 and 26 are designed to house the secondary windings 13 which are wound concentrically on a mandrel 36. The outer periphery of the mandrel 36 is closed by a cover constituted by a cylindrical ring 37. The mandrel 36 and its cover 37 fit inside the compartments 22 and 26. The second annular compartments 23 and 27 have twenty-four cells A1, A2, A3, . . . , A15, A15, A16, A24 which are designed to house respective ones of the twenty-four capacitors C1 to C24.
The third compartment 24 of the half-shell 20 is designed to house the diodes D1 to D24 and to provide the connections therebetween, their connections with the capacitors C1 to C24, and their connections with some of the output terminals of the secondary windings S1 to S12. This disposition is described in greater detail below with reference to FIG. 4.
The third compartment 28 of the half-shell 21 is designed to provide the various connections between some of the output terminals of the secondary windings S1 to S12 and the capacitors C1 to C24 as described below with reference to FIG. 5.
Each of the annular compartments 24 or 28 is closed by a respective annular cover 40 or 41 which fits around the outer periphery of the associated compartment. In order to enable the magnetic circuit 35 to be disposed close to the secondary windings, the periphery of each of the half-shells 20 (or 21) is interrupted by a notch 42 (or 43) and the same applies to each of the covers 40 (or 41). The notch serves to pass one of the branches of said magnetic circuit.
As shown in FIG. 4, the diodes D1 to D24 are disposed on a printed circuit in the form of a sector of an annular plate which provides the connection therebetween, the connections with one of the ends of each of the diodes C1 to C24, and the connections with the output terminals B1, B3, . . . , B23 in accordance with the circuit diagram of FIG. I. Thus, by way of example, the diode D1 has its cathode connected to the terminal B1 of the winding SI and its anode connected to one of the ends of the capacitor C1. In addition, the terminal B1 is connected to the anode cf the diode D2 the cathode of which is connected firstly to the anode of the diode D3 and secondly to one terminal of each of the capacitors C2 and C3, with the connection to the capacitor C3 taking place via a printed conductor CI1. It may be observed that the other printed conductors CI2 to CI11 connect other points common to diodes equivalent to D2 and D3 to capacitors equivalent to C3.
FIG. 5 is a plan view with the cover 41 partially cut-away, showing the other side of the secondary windings. In this figure, only connection conductors CC5 to CC10 between the terminals B6, B8, and B10, and the associated capacitors (C5, C6), (C7, C8), and (C9, C10) are shown. Of course, the conductors CC5 to CC10 may be made in the form of a printed circuit analogous to the printed circuit 38 carrying the diodes, or else in the form of a strip.
FIG. 5 also shows three of the four branches of the magnetic circuit 35 which has one of its branches housed in the notch 43.
The various components described above with reference to FIGS. 1 to 5 are assembled by being fitted within one another and they are held together by assembly components so as to obtain the assembly shown in partial section in FIG. 6. The assembly elements (not shown in FIGS. 1 to 5) are constituted by threaded tie bars together with nuts and support plates for holding the various branches of the magnetic circuit 35.
Thus, the components shown in FIG. 2 are held together by two threaded tie bars with nuts such as those referenced 50, 51, and 52 (FIGS. 2 and 6), with the tie bars being housed in holes 53 and 54 (FIG. 5) passing through the components shown in FIG. 2 from one side of the assembly to the other along axes parallel to the axis x'x.
Further, in order to support and hold the magnetic circuit 35 in place, plates 55 and 56 are provided (FIGS. 2 and 6) with these plates being held against respective ones of the covers 41 and 40 b/ threaded tie bars and nuts such as those referenced 57, 58, and 59 in FIGS. 2 and 6. Each of these plates 55 and 56 is designed to house and hold one of the branches of the magnetic circuit. Thus, the plate 55 supports the branch 60 of the U-shaped portion whereas the plate 56 supports the branch 46 of the magnetic circuit for closing the opening of the U-shape.
The device shown in FIG. 6 is placed inside an enclosure 61 (FIG. 7) filled with an insulating cooling fluid. To this end, it is mounted on a support plate 62 which constitutes the cover of the enclosure 61. It is mounted on the support plate 60 by means of two feet 63 and 64 which co- operate with retaining plates 55 and 56 by engaging in housings (not shown) provided therein. These feet 63 and 64 are pierced by holes such as the hole referenced 65 through which screws (not shown) are passed and then engaged in tapped holes in the cover 62.
The cover 62 also supports an insulating insert 66 which supports the high-voltage output terminal of the power supply. The other electrical terminals of the power supply are not shown in FIG. 7.
In spite of the considerable reduction in the bulk of the power supply, the enclosure 61 must nevertheless be sufficiently large to contain a large volume of cooling liquid, about 15 liters to 20 liters, which volume gives rise to a high-voltage power supply assembly which is relatively bulky.